Details
We have several cases of non-functional SIP lines on modem (RJ11).

The first investigations indicate that modems are registered (REGISTER) to the SIP infrastructure,
but they answer "USER BUSY" when requested "INVITE".
The problem seems localized to the modem SIP stack, we continue to investigate.

Meanwhile, in case of problems, a reboot of the modem allows to restart the modem SIP stack.